I understand that the mining difficulty of Bitcoin is controlled by the number of leading zeros in the 24-digit block solution. Now if one assumes no leading zeros and all 24 digits have to be brute-forced in order to get the solution, it would be considered maximum mining difficulty.
This assumption is incorrect. Miners attempt to find a hash that is smaller than a target value, so having at least a given amount of leading zeroes. The higher the difficulty, the lower the target value and the higher the number of leading zeroes.